The present invention relates to digital printing systems, in which images based on electronic image signals are printed. More specifically, the present invention relates to a halftone screen which allows for the printing of colors having substantially opposite hues in order to extend the color gamut, generate improved neutral colors, and smooth transitions through color space.
FIG. 1 depicts a representative "slice" of color space in the visible spectrum, showing the additive and subtractive primary colors. As is known in the art of color science, the slice illustrated in FIG. 1 is a section of a three-dimensional color space, with a white-to-black neutral axis emerging from the center of the diagram out of the page.
Around the perimeter of the section of color space are shown locations representing a full saturation of the subtractive primary colors yellow (Y), magenta (M), and cyan (C). These subtractive colors, as is well known, are used in the printing of images, because combinations of these colors can theoretically simulate all other colors in the visible spectrum.
Located between the various pairs of subtractive primary colors in the color space are what are here called the "hi-fi" colors, blue (B), red (R), and green (G). As can be seen in FIG. 1, each hi-fi color can theoretically be simulated by combining, such as on a printed surface, colorants (such as toner or ink) of the two adjacent primary colors, so that magenta plus cyan together on a printed sheet would yield blue, while cyan and yellow would produce green, and so forth. However, this theoretical mixing of primary colors to yield other colors may result in a limited printer gamut. The inevitable chemical shortcomings of typical colorant compositions will often cause combinations of primary colorants to yield a sub-optimal rendering of the desired combination color.
Within the shape shown in FIG. 1, wherein colors of any kind will be more vivid (i.e., have higher chroma) as one approaches the perimeter, the shaded area bounded by solid curved lines represents a typical practical gamut of colors obtainable with the printing apparatus. If it is attempted to print one of the primary colors, such as yellow, a yellow colorant is applied to the sheet, unalloyed with any other color; in such a case, pure yellow colorant will yield the theoretical maximum chrominance of the desired color. This optimal use of uncombined primary color is represented by the fact that the solid curved line within the shape substantially meets the perimeter of the shape only at the point of pure color, when yellow colorant is not combined with any magenta or cyan colorant.
However, if it is desired, for example, to print a green area, there must be supplied onto the paper a visually-effective combination of yellow with cyan. As long as one primary color dominates, almost optimal chrominance can be achieved, as is shown by the fact the curved solid line is reasonably close to the shape when yellow or cyan dominates. When colors toward a pure green are desired, which would require close to a half-and-half split of the two types of colorant, the lack of chrominance becomes noticeable, as shown in FIG. 1 by the fact that, near the area marked G, the solid curved line is quite far from the corner of the shape which represents a perfect green. In practical terms, the fact that the solid curved line is far from the perimeter of the shape results in a distinct dull or grayish appearance when the combination color is attempted. A similar lack of chrominance will appear when other hi-fi hues, such as red or blue, are attempted to be printed with close-to-equal proportions of subtractive colors.
It has been proposed, particularly in the art of xerographic printing and other ink jet and acoustic ink printing, to overcome the problem of obtaining the hi-fi hues by providing a printing apparatus which lays down not only the CMY primary colors, but also one or more apparatus (such as development units) which lay down one or more types of colorant to print hi-fi hues, such as RGB. Instead of trying to obtain, for example, pure blue by mixing magenta and cyan colorants, such a system would simply lay down a dedicated blue colorant. With the addition of a black (K) development unit such as for printing of text, improving an achievable optical density, and to help create neutral grays, such "hi-fi" color printing systems would typically include five or more development units.
Color printing on halftone printers involves the formation of color separations as halftone screens for each color which is to be used to form a color image. The halftone screens are laid down in a predetermined overlapping relationship to each other which result in generation of the desired color image. A well known problem when overlapping two or more halftone screens is the possibility of developing a moire pattern or other form of interference, when the screens are not properly positioned. To avoid the moire or other undesirable patterns, precise angle combinations of the screens are required. It is known that increasing the difference in an angle of two overlaid screens will result in a smaller pattern, making the pattern less apparent. It is noted that a 90.degree. screen is essentially the same as one at 0.degree., just as a 135.degree. screen is the same as a 45.degree. screen (though with asymmetrical dot shapes, the orientation of the dots varies around the full 360.degree. arc, - - - this is not a major factor in moire patterning). Thus, the largest possible angle difference between two overlaid screens is 45.degree..
If a two-color print is to be generated, the angles of the two color screens should be separated by 45.degree.. The dominant color is normally located at 45.degree. since it will be less apparent, and the secondary color - - - often black, at 0.degree.. In consideration of the foregoing, when three screens are used in a printing process, the maximum angle difference offset is 30.degree..
While images of two or three colors arc at times used, it is also common that four process colors are used in color image printing. The printing industry has therefore, generated a standardized combination of four halftone angles. In particular, cyan is located at 15.degree., black at 45.degree., magenta at 75.degree. and yellow at 0.degree.. Since yellow is the lightest and least noticeable color, it can be set at 0.degree., even though 0.degree. is a highly noticeable angle, and it is only 15.degree. from the nearest neighbor. In some embodiments, cyan is known to be set at 105.degree., however, with symmetrical dots this is substantially the same as 15.degree. (and even with asymmetrical dots, it does not make a large difference).
When four process colors using the above angle combinations are overlaid, the resulting moire or other interference patterns arc as small as possible. However, if these angles are off even a slight amount, problems with the image will occur.
It is known that many color printing systems will include five or more development units having different color colorants. Attempting to incorporate these additional colors is difficult, especially if each color must have a unique halftone angle. Particularly, once there are more than four angles, which must be laid down, the patterning problems discussed above are greatly increased. A known solution to limiting the angles which are incorporated in the printing process is to use the same halftone angle for colors at opposing hue regions such as red or orange used at the same angle as cyan, green used at the same angle as magenta, blue used at the same angle as yellow, and black used at the same angle as gold or silver.
A limitation placed on the use of the same halftone angles for colors of opposing hues is that the opposed colorants are not to be simultaneously printed. This constraint exists in order to prevent registration errors which degrade the output print.
It has been considered desirable to determine a manner which would allow for a relaxation of the limitation against a simultaneous printing of opposed colorants. In particular the present invention is directed to an arrangement which relaxes the constraint that the two opposed colorants be mutually exclusive at any given pixel location.
There are numerous teachings for converting a color signal in one color space to another color space. Such processing are understood to use halftone screens for color printing. A sampling of such teachings are provided below, and are hereby incorporated by reference.
In the prior art, U.S. Pat. No. 4,275,413 discloses a linear interpolation method for locating outputs of a three-dimensional look-up table, such as to convert a desired color from RGB to CMY color space.
U.S. Pat. No. 4,500,919 discloses a basic technique for obtaining a specific desired color from signals representative of various primary colors.
U.S. Pat. No. 4,812,899 discloses a printing technique in which the picture surface is divided into subsurfaces of identical size, with every subsurface divided into juxtaposed elemental surfaces which form a chromatic component and an achromatic component. The elemental surfaces which form the chromatic component are printed with a maximum of two of six chromatic printing inks, such as yellow, orange-red, magenta-red, violet-blue, cyan-blue, green and black.
U.S. Pat. No. 4,893,179 discloses a digital copier including a decomposing circuit for decomposing a color image into three fundamental colors. The original RGB data derived from the original decomposition is then converted to CMYK data for xerographically printing the image.
U.S. Pat. No. 5,047,844 discloses a color printing apparatus in which an edge portion of an achromatic area is detected to emphasize the edge portion and reduce the density of a chromatic area near the edge portion. This technique results in a reduction of color bleeding.
U.S. Pat. No. 5,077,604 discloses a method for converting RGB color separation signals into an equivalent CMYK image signals.
U.S. Pat. No. 5,087,126 discloses a method of estimating a combination of fundamental colors which corresponds to a target color desired to be printed.
U.S. Pat. No. 5,136,372 discloses a color xerographic printer. A spatial frequency detector detects a spatial frequency relating to an image of a prescribed color for every portion of an image to be formed. The images are formed with different xerographic techniques, depending on whether there is high spatial frequency of the image desired to be printed.
U.S. Pat. No. 5,140,411 discloses a color image reader, in which light from the original image is divided, by means of a prism, into separate components which can be fed to a discriminator for discriminating between a chromatic portion of the light image and an achromatic portion of the light image.
U.S. Pat. No. 5,208,663 discloses an image processing apparatus in which color image data is classified as including either an achromatic color, a chromatic color, or an intermediate color. The apparatus further includes a discrimination circuit for discriminating a kind of the original image on the basis of the color image data, with a classifying circuit being capable of changing the classifying criterion in accordance with the discriminated kind of the original image.
U.S. Pat. No. 5,510,910 discloses a technique of merging color signals to map control signals for a CRT through a common perceptual space into printer control signals.
U.S. Pat. No.5,528,386 describes an apparatus for taking an original RGB image and converting the signals therefrom to a CMYK image which can be fed to a printing apparatus.
Ostromoukhov, "Chromaticity Gamut Enhancement by Heptatone Multi-Color Printing," SPIE, Volume 1909, page 139, June, 1993, gives an overview of the basic techniques of extending a CMYK printing process to a CMYKRGB printing process.
Boll, "A Color to Colorant Transformation for a Seven Ink Process," presented at the IS&T-SPIE Symposium on Electronic Imaging, Science and Technology, February 1994, discloses the selection of a primary color to obtain desired color in a CMYKRGB apparatus. The disclosed technique subdivides the gamut formed by the seven possible colorants into smaller groupings. A series of four-colorant subsets from the seven-ink superset of CMYKRGB are individually characterized and a calorimetric transform was obtained for each subset. In color space each of the four-colorant subsets represent adjacent and overlapping subgamuts of the seven-colorant gamut.
The article "New Era of Digital Photo Printing . . . ", Hard Copy Observer, October 1996, p.1, and its ancillary articles, discloses currently popular techniques for gamut enhancement, particularly in regard to ink-jet printing. Among these techniques are using primary color inks of different densities (e.g. a dark cyan ink and a light cyan ink), or adding orange and green primary inks (this is known as the Pantone "hexachrome" system).
U.S. patent application Ser. No. D/96095 entitled System for Printing Color Images With Extra Colorants In Addition to Primary Colorants.
U.S. patent application Ser. No. D/96099 entitled System for Printing Color Images With Extra Colorants in Addition to Primary Colorants.
SUMMARY OF THE INVENTION According to one aspect of the present invention, there is provided a printing apparatus capable of printing multi-color images. The printing apparatus uses halftone screens for each of the colors used in the printing operation. A first halftone screen is provided including a plurality of dots, where the first halftone screen is also arranged at a first angle. A second halftone screen also having a plurality of dots is then formed and also arranged at the first angle, and further is inverse to the first halftone screen. The inverse halftone screen is offset from the first halftone screen whereby the dots of the inverse halftone screen are midway between centers of two dots of the first halftone screen. In accordance with another aspect of the present invention, the printing apparatus allows for a simultaneous printing of colors having opposite hues at substantially the same pixel area.
A first benefit of the present invention is found in extending the color gamut of which an image may be printed. A second benefit of the present invention permits simultaneous printing of colors of opposing hues at the same screen angle without mis-registration artifacts. A further benefit of the present invention allows for the generation of high-fidelity grays by the combination of colors having opposing hues. Still yet another advantage of the present invention is allowing a smooth transition through the color space from a first color hue to an opposing color hue whereby a black application of color is not necessary for the intermediate gray area.